1. Field of the Invention
The present invention relates to a method of manufacturing a phase-shifting mask used in fabricating an LSI device. It particularly relates to a method of forming a phase-shifted pattern with high accuracy.
2. Description of the Background Art
When a reduction type stepper is used in photolithography, a resolution limit R is: EQU R=K1.lambda./NA
where K1 is a constant dependent on a type of a resist, .lambda. is a wavelength of an exposure light beam and NA is a numerical aperture of a lens. The lower limit of the constant K1 is around 0.5.
The resolution limit R is lowered, in other words, resolution is enhanced, by decreasing the constant K1 and using smaller wavelength .lambda. while increasing the numerical aperture NA. It is known that, NA can be reduced to around 0.5. Hence, where NA is around 0.5 and the i-beam (.lambda.=365 nm) is used, the resolution limit R as low as around 0.4 micron is possible.
To further lower the resolution limit R, either much shorter wavelength .lambda. or much larger NA is necessary. However, both a light source producing such a short wavelength and a lens having such a large NA should require extremely difficult structure. Rather, a much shorter wavelength .lambda. and much larger NA have adverse affect. When the wavelength .lambda. is further decreased or when NA is further increased, a depth of focus .delta. is reduced, damaging the resolution, since the depth of focus .delta., the wavelength .lambda. and NA are in the following relation: EQU .delta.=.lambda./[2(NA)2]
Various methods have been proposed to solve the problems. One such effort is phase-shifting lithography exposure as disclosed in Japanese Laid-Open Gazette Nos. 57-62052 and 58-173744. Advantages of phase-shifting lithography over lithography using an ordinary photomask will be described below.
FIGS. 1A-1C are diagrams showing principles of lithography using an ordinary photomask. The ordinary photomask comprises a transparent glass substrate 1 with an opaque light-shielding pattern 2 formed thereon. The light-shielding pattern 2 consists of light-shielding regions and apertures 2a (FIG. 1A). In the case of molybdenum silicide, however, any other material may be used which blocks exposure light beams used in lithography such as the g-beam, the i-beam and excimer laser beams.
When a light beam is transmitted through the ordinary photomask, light intensity at a wafer is largely affected by interference as shown by the intensity waveform of FIG. 1C. At the plane immediately below the photomask, electric field is zero in magnitude except at the apertures 2a and is otherwise constant (FIG. 1B). However, at the wafer, which is a certain distance away from the photomask, the intensity does not remain zero at the same regions, that is, regions corresponding to the light-shielding regions 2a (non-transparent regions) since light beams diffracted from the adjacent apertures overlap and intensify each other. Hence, the pattern 2 cannot be resolved.
FIGS. 2A-2C are diagrams showing principles of phase-shifting lithography. A phase-shifting mask of FIG. 2A is obtainable from adding a phase-shifter 3 made of transparent material such as SiO.sub.2 to the ordinary photomask in such a manner that the phase-shifter 3 covers all other apertures. In this phase-shifting mask, a light beam transmitted by the phase-shifter 3 is 180 degrees out of phase with an incident light beam. The phase-shifting mask is otherwise identical to the ordinary photomask.
Assume that a light beam is transmitted through the phase-shifting mask. At the plane immediately below the mask, the electric field passing through the covered apertures has a sign opposite to that of the electric field passing through the uncoated apertures (FIG. 2B). Therefore, when the pattern 2 is projected onto a wafer by the same projection optical system used in the ordinary lithography, light beams diffracted from the adjacent apertures cancel out each other at the wafer at the non-transparent regions. As a result, at the regions where cancellation occurs, intensity becomes practically zero, whereby the pattern 2 is transferred onto the wafer with high resolution. According to experimental results, a minimum width of a pattern resolved by using the phase-shifting mask of FIG. 2A is about half that of a pattern resolved by using the ordinary photomask of FIG. 1A.
The phase-shifting mask of FIG. 2A, however, is insufficient. Although excellent for periodic line-and-space pattern implementation, the phase-shifting mask is not applicable, if not at all, to where the pattern 2 is an optional pattern. Against this backdrop, improved phase-shifting masks have been proposed. For example, the mask described in the article "NEW PHASE SHIFTINGMASK WITH SELF-ALIGNED PHASE SHIFTERS FOR A QUARTER MICRON PHOTOLITHOGRAPHY," A. Nitayama et al., IEDM Conference, 1989.
The improved phase-shifting mask disclosed in the article is obtained by adding to the ordinary photomask of FIG. 1A a phase-shifter 3 which is a little wider than the light-shielding pattern 2 such that the phase-shifter 3 is formed on the light-shielding pattern 2 (FIG. 3A).
The phase-shifter 3 is formed from a resist pattern which functions as an etching mask when the light-shielding pattern 2 is etched. Specific manufacturing process of the phase-shifter 3 is as follows: First, a light-shielding film is deposited on a glass substrate 1. Next, a resist pattern, i.e., the phase-shifter 3 is formed on the light-shielding film. The light-shielding film is then patterned by anisotropic etching such as plasma etching with the phase-shifter 3 as a mask, thereby obtaining the light-shielding pattern 2. Furthermore, edge portions of the thus obtained light-shielding pattern 2 are removed a little bit by isotropic etching such as wet etching. As a result, the light-shielding pattern 2 which is a little narrower than the phase-shifter 3 is formed (FIG. 3A).
When a light beam is transmitted by the phase-shifter 3, light intensity at a non-transparent region disappears at a wafer (FIG. 3B). Hence, the light-shielding pattern 2 is resolved with high accuracy, printing a very sharp pattern on the wafer. In addition, since the phase-shifter 3 is formed easily in a self-aligned manner, this improved phase-shifting approach promises accurate pattern transfer even when the light-shielding pattern 2 has a non-periodic feature.
Recent efforts have resulted in further improvement in the phase-shifting approach as above. One such effort is phase-shifting in which a phase-shifting mask produces a null or a sharp drop in intensity at an edge of the phase-shifter 3. This approach is disclosed, for example, in the article "0.2 .mu.m OR LESS i-LINE LITHOGRAPHY BY PHASE-SHIFTING-MASK TECHNOLOGY," H. Jinbo et al., IEDM Conference, 1990. According to the disclosed phase-shifting technology, the phase-shifter 3 is formed directly on the glass substrate 1. A light beam transmitted at an edge portion 3b of the phase-shifter 3 is therefore 180 degrees out of phase with an incident light beam (FIG. 4B), whereby light intensity at the edge portion 3b partially drops to zero at the wafer (FIG. 4C). Hence, a fine resist pattern is printed. The article reports that a 0.15-micron wide isolated space is resolvable by exposing a negative resist using the phase-shifting mask of FIG. 4A.
Still further improvement has been disclosed by the article "A New Phase Shifting Mask Structure for Positive Resist Process," J. Miyazaki et al., Proceeding of SPIE's 1991 Symposium on Microlithography. The article introduces a phase-shifting mask which is obtainable by modifying the phase-shifting mask of FIG. 4A such that the thickness of the edge portion 3b is thinner around one end than around the other end (FIG. 5A). In the modified phase-shifting mask, a light beam transmitted at the edge portion 3b in the thinner side arrives at a wafer about 90 degrees out of phase with a light beam through an unmasked region while light beams at the other portions arrive about 180 degrees out of phase. Thus, the light intensity at the end portion 3b is almost the same as that at the unmasked region although the null or the sharp drop in intensity at the other end portions occurs (FIG. 5B).
Now, manufacturing process of the phase-shifting mask of FIG. 2A will be described to show where inconvenience lies in the phase-shifting masks heretofore described. First, a light-shielding film made of molybdenum silicide is formed on the glass substrate 1. The light-shielding film is then patterned by electric-beam lithography (EB lithography) to define the light-shielding pattern 2. Next, an SiO.sub.2 film is deposited on the substrate-pattern combination. The SiO.sub.2 film is then patterned by EB lithography, thereby the phase-shifter 3 being formed. Thus, the phase-shifting mask of FIG. 2A is obtained.
The phase-shifting mask of FIG. 4A is obtainable by the same manufacturing process as above only if the second EB lithography patterns the SiO.sub.2 film in such a manner that the SiO.sub.2 film remains partially unetched on the glass substrate 1 in order to form the edge portion 3b.
The phase-shifting mask of FIG. 5A is obtainable by further modification; still more EB lithography reduces the thickness of the edge portion 3b of the phase-shifger 3.
All of the manufacturing processes require EB lithography at least twice. The second EB lithography is particularly laborious because the phase-shifter 3, which will be defined by the second EB lithography, must be aligned with extremely high accuracy to the light-shielding pattern 2, which has been already defined by the first EB lithography. Despite such requirement, since relative positional accuracy between the phase-shifter 3 and the light-shielding pattern 2 is dependent on the drawing accuracy of an E-beam, it is impossible to improve the positional accuracy beyond what the drawing accuracy enables.
The substrate 1 is made of the same material as the phase-shifter 3. When etching time is over an optimum period, not only the SiO.sub.2 film (phase-shifter) but also the substrate 1 is etched. In reactive ion etching (RIE), furthermore, the glass substrate 1 becomes charged, and the etching accuracy is degraded.
Another problem encountered during the manufacturing processes of the phase-shifting masks is deviation of E-beam due to electrification of the glass substrate 1. The glass substrate 1 is insulative, and hence, is charged when illuminated with E-beam. As a result, the E-beam would deviate and hit the phase-shifting mask at a wrong location, which means that pattern definition becomes different from what is desired.